As the density of semiconductor devices increases and the size of circuit elements becomes smaller, the resistance capacitance (RC) delay time increasingly dominates the circuit performance. To reduce the RC delay, the demands on interconnect layers for connecting the semiconductor devices other increases. Therefore, there is a desire to switch from traditional silicon dioxide based dielectrics to low-k dielectrics. These materials are particularly useful as intermetal dielectrics, IMDs, and as interlayer dielectrics, ILDs.
One example of a low-k material is fluorine-doped silicon dioxide, or fluorosilicate glass (FSG). Another widely used material is a carbon-doped oxide or organosilicate glass (OSG). OSG films typically comprise SiwCxOyHz wherein the tetravalent silicon may have a variety of organic group substitutions. A commonly used substitution creates methyl silsesquioxane (MSQ), wherein a methyl group creates a SiCH3 bond in place of a SiO bond. There are several approaches known in the art for reducing the k-value of dielectric films. These include decreasing the film density, reducing the film ionization, and reducing the film polarization.
Since air has a dielectric constant of about 1, one method for making low-k dielectrics incorporates air into dense materials to make them porous. The dielectric constant of the resulting porous material is combination of the dielectric constant of air and the dielectric constant of the dense material. Therefore, it is possible to lower the dielectric constant of current low-k materials by making them porous.
Silica based xerogels and aerogels, for example, incorporate a large amount of air in pores or voids, thereby achieving dielectric constants less than 1.95 with pores are as small as 5–10 nm. A major drawback with low-k dielectrics, however, is that they are susceptible to damage from plasma etching and ashing processes used in device fabrication. In etch and ash processing, low-k materials frequently suffer from carbon depletion at the surface exposed to the plasmas. In certain etch and ash processes, the damage may also extend into the bulk as well. When there is an open pore structure in the dielectric, processing fluids in lap and polish and in thin film metallization can enter surface pores, thereby causing corrosion, mechanical damage, or an increase in the dielectric constant. Pore damage may also cause a surface that is preferably hydrophobic to become hydrophilic.
Recognizing the need to seal open pores near the surface of dielectrics, methods have been developed to deposit films on dielectric surfaces thereby sealing the open pores. However, some techniques are problematic in that highly surface-conforming layers may actually penetrate into the pore cavity. In such cases, even an insulating, pore-sealing layer defeats the advantage of the porous, low-k material by raising its dielectric constant.
Accordingly, a need exists for more effective methods of sealing pores in low-k dielectrics, particularly in the context of dual damascene metallization.
In addition to sealing pores, there is the need to repair damage to ILD's caused by plasma processing. Such plasma processes include etching, including etching of the low-k film, removing photoresist masking material, and depositing layers in plasma-enhanced chemical vapor deposition (PECVD) processes. In etch and ash processing, low-k materials frequently suffer from carbon depletion at the surface exposed to the plasmas. In certain etch and ash processes, the damage may also extend into the bulk as well. Upon subsequent exposure to air, or even in an oxygen containing ash, these damaged surfaces may react with moisture to form silanol groups (≡Si—OH) at free Si sites. The silanol group is known in the art to increase the dielectric constant of the low-k dielectric material. It is also known that the damaged low-k dielectric material is vulnerable to chemical attack during exposure to wet chemical cleanups, mainly those containing HF or related compounds like NH4F, which results in significant critical dimension (CD) loss of low-k dielectric film insulating structures. Similar effects are believed to occur in other low-k dielectric materials with silicon-hydrocarbon bonds that are converted to silanol when exposed to oxidizing or reducing plasmas.
Therefore, semiconductor manufacturers need a method for repairing carbon depletion in low-k dielectrics. Also needed is a method to seal pores in porous low-k dielectrics.